1. Field of the Invention
This invention relates to organosiloxane compositions containing a uniformly dispersed solid phase. More particularly, this invention relates to organosiloxane compositions containing a thixotropic matrix in the form of a liquid, paste or gel and a suspended solid phase that does not precipitate during storage of the compositions for extended periods of time. When subjected to shearing forces the matrices in the form of pastes and gels convert to flowable liquids.
2. Background Information
One of the problems associated with low viscosity organosiloxane compositions containing pigments or other insoluble solid materials having a higher density than that of the liquid phase is the tendency of the solid phase to settle during storage of the compositions. When the solid is a pigment and compositions are fabricated into shaped articles by molding or extrusion the tendency of the pigment to separate can result in uneven coloration of the final product.
The prior art discloses a variety of additives yielding stable dispersions of pigments and other solids in liquid compositions.
The use of colloidal silica as a pigment dispersant in latex compositions is described in Japanese Laid Open Patent Application No. 4/110,302. In accordance with the teaching of this patent, a polymer emulsion with a glass transition temperature of up to 0.degree. C. is blended with from 0.5 to 10 weight percent of colloidal silica. A mixture of an unsaturated organic monomer and a vinyl-functional silane is then polymerized in the presence of the emulsion. The resultant composition is used as a pigment dispersion vehicle.
U.S. Pat. No. 5,006,597 describes a storage stable water-based black corrosion resistant top coat containing from 1 to 40 weight percent of an aqueous silica sol and at least 0.2 weight percent of amorphous silica.
Commonly assigned U.S. Pat. No. 5,036,121, which issued to Alan Himstedt on Jul. 30, 1991 describes moisture curable thixotropic dispersions containing a hydroxyl-terminated polydimethylsiloxane, a moisture-activated crosslinking agent for this polymer, a catalyst for the crosslinking reaction, a hydrated aluminum oxide and, as the thixotropic agent, a mixture of an untreated silica and a hydroxyl-terminated polydiorganosiloxane wherein at least a portion of the silicon-bonded hydrocarbon radicals are phenyl or 3,3,3-trifluoropropyl.
Thixotropic agents that have been used in curable organosiloxane compositions include silica treated with various organosilicon compounds and organosiloxane/oxyalkylene copolymers.
To achieve adequate storage stability some curable organosiloxane compositions, particularly those containing tin compounds as the catalysts for the crosslinking or curing reaction, are packaged with the tin compound and crosslinkable polyorganosiloxane in separate containers. The crosslinking agent is typically packaged together with the curing catalyst.
Because only a small volume of crosslinking agent relative to the volume of crosslinkable polyorganosiloxane is required to achieve the desired degree of crosslinking, the portion of the composition containing the crosslinking agent typically contains a quantity of diluent that will allow this part of the composition to be blended with the part containing the crosslinkable polyorganosiloxane in a volume ratio of 1 part of curing agent portion to up to about 20 parts of the portion containing the crosslinkable polyorganosiloxane. The diluent is a solvent for the crosslinking agent and the catalyst.
Commonly assigned U.S. patent application Ser. No. 08/401,104, filed on Mar. 8, 1995 and now abandoned, describes two-part, moisture curable organosiloxane compositions. The portion of these compositions containing the crosslinking agent and curing catalyst is thixotropic and comprises, in addition to the crosslinking agent and catalyst, a specified class of liquid diorganosiloxane/oxyalkylene copolymers as the thixotropic agent together with a specified class of non-reactive organosiloxane oligomers containing silicon-bonded phenyl radicals as a diluent. The examples in this application describe preparation of the thixotropic portion of the two-part composition by combining all of the ingredients of this portion together, blending these ingredients to homogeneity and combining the resultant mixture with the other ingredients of the curable composition.
It was subsequently found that when prepared as described in the examples of the aforementioned application, the thixotropic portion of the two-part composition separated into a very soft gelatinous material and a supernatant liquid. When this two-phase thixotropic portion contained a pigment that had a higher density than the density of the liquid, a portion of the pigment settled to the bottom of the liquid phase.
When the pigmented portion containing the catalyst was combined with the base portion comprising a crosslinkable liquid polyorganosiloxane and fillers, the pigment could not be homogeneously blended into the resultant mixture to form a uniformly pigmented, homogeneous composition.
One objective of this invention is to provide uniform dispersions of finely divided solids, including but not limited to pigments, in low molecular weight organosiloxane compositions that will not settle upon standing, and a method for preparing these compositions.
A second objective of this invention is to provide multi-part moisture curable organosiloxane composition wherein the portion containing the crosslinking agent also contains a stable dispersion of a finely divided solid exhibiting a higher density than that of the organosiloxane compounds present in the composition.
Another objective is to provide a method for preparing organosiloxane compositions containing a dispersed form of a finely divided solid that does not settle during long term storage of the compositions.